Empty lines inside table cell cause empty WW result

[ibedarev]

Here is br tag causes stack.pop https://github.com/nhn/tui.editor/blob/0c5c11bac0b9954e2cac2ff2a081ce8c80a5f14d/apps/editor/src/convertors/toWysiwyg/htmlToWwConvertors.ts#L186

After convert cycle stack will be empty and converter will return null as result

Md example

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